There is a lot of engineering in the new toilets to get them to work properly with the much smaller volume of flush water. Check some of the reports on how well some of them work. Not all toilets are created equal, either. SOme of the models from better known US brands have lousy performance, so be careful in choosing a new one.

No, using less water per flush is not likely to work with an older design, and you'd have trouble (probably) getting a new tank to fit or match. Some companies have as 2-3 or more white porcelain glazes, and one to another often won't match.

Just my humble opinion after trying to "upgrade" one (!) toilet to low flow: I would never, ever do it again unless I have to. The quality of older toilets appears to be magnitudes better than some of the "new and improved".

The old bowls need more water to clear the bowl.
I've seen a new tank on an old bowl, and it just stirred the water, it didn't flush it.
Toto makes some very nice low-flow toilets with classic styles that would work very well in commercial usage.

Joerg, It's too bad about your situation.
I've installed plenty of the Glenwalls, from American Standard that worked fine.

It sounds like you got a defective bowl on your American Standard Yorkville.

I would have them replace it with a tested bowl from the factory.

I sell about 2000 new toilets a year, and if I had those kind of problems, I would find something better to do.

Thank goodness, that most of the customers use standard floor mount toilets, which tend to work much better than the rear outlet kind.

I also sell mainly Toto, which maintains excellent quality controll.
It's too bad they don't make a rear outlet toilet that would work in your situation too.